The masseter muscle of the pig is proposed as a model to study neuromuscular development and aging in complex muscles. Such a model is also appropriate for studies on peripheral nerve disease and regeneration of nerve and muscle after traumatic injury. Like other multipinnate muscles, the pig masseter has an intricate pattern of contraction with varying and function-related activation of fibers in different parts of the muscle. Three central questions are asked. (1) What is the underlying neural organization which gives rise to the contraction pattern? Motor unit territories are hypothesized to be small and arranged to corespond with the observed contraction pattern. The hypothesis will be tested by motor unit mapping via glycogen depletion, dissection of the masseteric nerve to determine gross branching, and fiber counts at various levels of the nerve to determine axon branching. (2) When does the pattern arise ontogenetically and how does aging perturb it? The hypothesis that neural organization is secondary to the nerve branching pattern will be tested by repeating the above procedures on neonatal and aged animals. An additional hypothesis that the neural organization is not used to produce complex contractions until mastication begins in the second month will be examined by multielectrode elctromyography. (3) What are the determinants of the branching pattern of the nerve? It is hypothesized that the developing nerve grows into a muscla anlage that is already a complex of fibroblasts and myoblasts, and branching is directed accordingly. Testing will be by observation of the differentiation of myoblasts and fibroblasts, as indicated by the production of myofilament proteins and other markers, in staged pig embryos.